Thibault Taillandier-Thomas

Slant strained band development during flat to slant crack transition in AA 2198 T8 sheet: in situ 3D measurements

In this work 3D strain and damage analyses are performed in the immediate vicinity of the notch root of a flat CT-like specimen made of aluminum alloy. Experimental data, partially exploited by Morgeneyer et al.xa0(Acta Mat 69:78–91, 2014b), were obtained by using synchrotron laminography and the 3D reconstructed volumes are subsequently analyzed via Digital volume correlation. These data enable for in situ assessments of strain fields and ductile damage in the zone where the stress triaxiality evolves from elevated to lower levels, which is accompanied by flat-to-slant crack transition. The measured strain field patterns in this area are analyzed herein in a systematic manner by studying the incremental strain activity during several loading steps. It is shown that from the very beginning of the loading history multiple slant strained bands appear in front of the notch root while the corresponding damage growth sets in at later loading stages and higher strains. The activity of the different strained bands at the notch root is alternating between different locations over the loading history. However, the band leading to final rupture is always active. The region where slant fracture occurs is identified to be in plane strain condition with respect to the crack propagation direction.

Soft Route to 4D Tomography

Based on the assumption that the time evolution of a sample observed by computed tomography requires many less parameters than the definition of the microstructure itself, it is proposed to reconstruct these changes based on the initial state (using computed tomography) and very few radiographs acquired at fixed intervals of time. This Letter presents a proof of concept that for a fatigue cracked sample its kinematics can be tracked from no more than two radiographs in situations where a complete 3D view would require several hundreds of radiographs. This 2 order of magnitude gain opens the way to a computed 4D tomography, which complements the recent progress achieved in fast or ultrafast computed tomography, which is based on beam brightness, detector sensitivity, and signal acquisition technologies.

Measurement of 3D displacement fields from few tomographic projections

The present paper aims at providing 3D volume images of a deformed specimen based on i) a full 3D image describing the reference state as obtained e.g., from conventional computed tomography and ii) the 3D displacement field accounting for its motion. The displacement field, which is described by much fewer degrees of freedom than the specimen volume itself, is here proposed to be determined from very few projections. The reduction in number of needed projections may be larger than two orders of magnitude. In the proposed approach, the displacement field is described over an unstructured mesh composed of tetrahedra with linear shape functions. The mesh is based on the reconstructed reference volume so that it provides a faithful and accurate description of the specimen, including its boundary. Nodal displacements are determined from the minimization of the quadratic difference between the computed projections of the deformed configuration and the acquired projections (radiographs) for the selected orientations. Well-posedness of the problem requires the number of kinematic unknowns to be small. However, in cases where the geometry is complex, the displacement field may call for many parameters. To deal with such conflicting demands it is proposed to use a regularization based on the mechanical modeling of the displacement field using a linear elastic description.

Evaluation of measurement uncertainties of digital volume correlation applied to laminography data

The effect of different data acquisition parameters on the displacement and strain uncertainties of kinematic fields is assessed via digital volume correlation applied to three-dimensional data sets imaged by synchrotron laminography. The measurement uncertainty is estimated for (1) different materials and associated varying contrast; (2) repeated scans, that is, uncertainty due to the imaging system and three-dimensional reconstruction; (3) rigid body motions between two subsequent scans; (4) rescaling of the gray level histogram during 32-bit floating point to 8-bit integer data conversion; (5) changes in the beam properties, that is, use of a monochromator or an undulator; and (6) changes in camera/detector characteristics. It is found that the amount of image contrast is not the only parameter that controls uncertainties for different materials. Furthermore, the rigid body motion procedure should be preferred over repeated scans as it provides more conservative measurement uncertainty values. The applied 32- to 8-bit conversion procedure, beam tuning and detector characteristics hardly affect the measurement uncertainty.

On the use of regularized DVC to analyze strain localization

For an in-depth understanding of the failure of structural materials the study of the deformation mechanisms in the bulk is fundamental. In situ synchrotron radiation computed laminography provides 3D images of thin plates subsequently processed by digital volume correlation to measure displacement and strain fields by using the natural contrast of the material. Difficulties arise from the lack of data, which is intrinsic to laminography and leads to several artifacts, and the weak absorption contrast in the 3D image texture of the studied material. To lower uncertainty levels and to have a better mechanical admissibility of the measured displacement fields, a regularized digital volume correlation procedure is introduced and applied to analyze localized displacement and strain fields.

Equilibrium Gap Regularized Radio-Based Digital Volume Correlation

The measurement of 3D displacement fields experienced by a sample that has been reconstructed in its reference state can be achieved from much less projections than needed to image the sample itself. The principle of the approach is discussed together with proposed extensions.